Interstellar Formation of Thioethanal (CH$_{3}$CHS). Gas-Phase and Ice-Surface Mechanisms involving Secondary Sulfur Products

Abstract

The formation pathways of sulfur-bearing species in the interstellar medium are crucial to understand astrochemical processes in cold molecular clouds and to gain new insights about the sulfur budget in these regions. We aim to explore the recently detected, thioethanal (CH$_{3}$CHS) formation mechanisms from thioethanol (CH$_{3}$CH$_{2}$SH) as a precursor in addition to secondary sulfur products. The electronic structure methods and density functional theory for both gas-phase and ice-grain surface environments is employed. To mimic interstellar ice-mantles, we use medium (W6) and large amorphized (W22) water clusters as implemented in Binding Energy Evaluation protocol. A barrierless formation mechanism for CH$_{3}$CHS under low-temperature interstellar conditions is identified, in the gas phase. Surface environments modulate activation barriers in a site-specific manner, elucidated through both Langmuir-Hinshelwood and Eley-Rideal initiated surface reaction pathways. Compared to oxygen analogs, sulfur chemistry enables alternate pathways due to weaker S-H bonding, with a competing route forming ethane-1,1-di-thiol (CH$_{3}$CH(SH)SH), on the ice-grain surface, potentially reducing CH$_{3}$CHS yields. The first accurate binding energy for thioethanol on water ice is also reported, confirming its greater volatility than ethanol. The proposed mechanism offers a tentative hypothesis for the apparent mutual exclusive detections of the CH$_{3}$CH$_{2}$SH and CH$_{3}$CHS in TMC-1, Orion, and Sgr B2(N), that further requires validation through quantitative astrochemical modeling and also to distinguish this chemical differentiation from observational sensitivity limitations. These qualitative findings highlight the multifaceted chemical behavior of sulfur-bearing organics in the interstellar medium and support CH$_{3}$CH(SH)SH as promising astro-chemical targets.

Figures (15)

• Figure 1: Representative structures of the water clusters used to investigate binding energies and reactivity. (a) One of the seventeen W22 clusters; (b) one of the three W6 clusters. Approximate dimensions are in Å. Oxygen is in red and hydrogen in white.
• Figure 2: Three possible pathways for hydrogen abstraction from CH$_3$CH$_2$SH by the OH radical: (a) abstraction from the -CH$_2$ group, (b) abstraction from the -SH group, and (c) abstraction from the -CH$_3$ group. All geometries are optimized using the M06-2X/def2-TZVP level of theory, with energies calculated at CCSD(T)/CBS, including zero-point energy (ZPVE) corrections at M06-2X/def2-TZVP. The ... represent bond formation and cleavage processes, with significant bond lengths indicated in Å. The atom color scheme is consistent throughout the paper and is illustrated separately for reference.
• Figure 3: Gas-phase reaction mechanism: (a) Step 2a, 2b and (b) Step 2c, CH$_3$CH(S)SH + H recombination, is shown only for comparison with the surface mechanism and is not expected to be feasible in the gas phase. Geometries are optimized at the M06-2X/def2-TZVP level of theory, and energies are computed using CCSD(T)/CBS with zero-point energy (ZPVE) corrections at M06-2X/def2-TZVP . Significant bond lengths are shown in Å
• Figure 5: (a) Types of binding modes observed on the W22 ice cluster surface and (b) corresponding binding modes on the W6 ice cluster. The binding interactions shown illustrate how adsorbate molecules adhere to different surface sites on both clusters. Key interatomic distances are shown (in Å), illustrating that W6 preserves the local orientation of the reactive groups. EtSH-I is used for modeling the LH-initiated pathways, while EtSH-II and EtSH-III are used for the ER-initiated pathways (details in the section \ref{['Selection of Reactive Binding Sites']}).
• Figure 6: Reaction pathways, W6$_{a1}$ and W6$_{a2}$ from the binding mode EtSH-W6-I. For (a) Step 1 and (b) Step 2a, 2b and (c) Step (c). The reactants are co-adsorbed for (a) Step 1. Geometries are optimized at the M06-2X/def2-TZVP and the energies are refined at MPWB1K-D3(BJ)/def2-TZVPD including zero-point energy (ZPVE) corrections at the optimization level of theory. Significant bond lengths are shown in Å. The bond length involved in making and breaking are depicted in red dotted lines